Thursday, December 15, 2016

Zombie fungi and ant graveyards

by JJIt is not unusual for parasites to control the behavior of their hosts, sometimes causing dramatic changes in their behavior: Nematode worms have been shown to cause their insect hosts to drown themselves so that their adult stages can reproduce in water 1. Some parasitic wasp larvae
can drive bumblebees to bury themselves alive to provide a better environment
for the developing larvae2. Similarly, the parasitic
fungus Ophiocordyceps unilateralis affects
its insect host’s brain, effectively turning the insect into a ‘zombie’ whose
only purpose is to serve as food for the fungus and a vehicle to carry it to more
favorable environments.

O. unilateralis was
first described by English scientist Tom Petch in 1931 as a top-shaped fungus
found on dead aphids3(A common sap-sucking insect
pest). He also noted how all dead aphids infected by the fungus were found
‘glued’ to leaves by fungal protrusions3. Since then, O. unilateralis has been shown to infect
many species of insects, but especially ants in tropical forests around the
world. There are even distinct species found in China, South America, and Africa4,5. These fungi are thought to help
keep insect populations in check. They have also been used in ancient Chinese
medicine for hundreds of years, although they have only recently been
investigated for the production of therapeutic compounds6.

When an ant is infected by O. unilateralis while foraging, it will eventually abandon all
ant-like behavior, its mind seemingly under the control of the fungus: hence
the term ‘zombie fungus’. When the ant walks over fungal spores, the spores
cling to the exoskeleton (a hard shell-like outer layer) of the ant. Within 3 days,
the spores germinate and reproduce within the ant7. In the meantime, the ant’s behavior starts to
change. It stops foraging and following scent trails altogether and starts to climb
to locations close to the forest floor, such as leaves. Once it reaches a leaf,
the ant uses its mandibles (mouthparts) to clamp on to the leaf and stays put
until it eventually dies of starvation, the fungus having used its body to
reproduce. It is not clear how the ant’s colleagues feel about this.

Once the ant dies, the
fungus produces a long stalk emerging from the back of the ant’s head (Fig. 1).
This stalk bears a structure filled with new fungal spores. Eventually, these
spores drop to the forest floor, where they can infect new insects and continue
the cycle. The spread of spores in this manner means infected insects are often
found clustered around the same area, forming ‘graveyards’7. Researchers have undertaken the grim task of
studying different conditions surrounding these graveyards over time and have
found that they are more likely to be present where humidity is high and
foliage is dense- conditions that would give the fungus a better chance to
reproduce. The number of graveyards has also been found to increase during the
monsoons, and decrease in the summers7. More importantly, other insects have been
shown to actively avoid these graveyards, and ant trails very rarely cut across
graveyards7. Ant colonies prefer foraging up the treetops
and minimize time spent on the forest floor7. They are also known to groom each other, which
is thought to help remove spores and keep the colony immune to infection8. These behaviors have probably evolved as defense
mechanisms against fungal infection, and suggest that the fungus has co-evolved
with insects, with insects being evolutionarily selected for avoiding the
fungus, and the fungus evolving to more efficiently disperse its spores5.There are other aspects of O. unilateralis infection that are yet
to be fully understood. For example, the interactions between the parasite and
the host at the molecular level are not yet clear9. These are important because they would give us
a clearer picture of how the fungus proceeds from the spore to the stalk-forming
stage during infection. Next, the effect that the fungus has on the insect
nervous system also needs to be described, aside from just behavioral changes. The
full range of behavioral changes induced in ants is also not clear: some
researchers suspect that the fungus induces aggressive behavior in ants, which
could help the spores spread to other workers in the ant colony9. Using large indoor ant colonies for
observational studies (Fig. 2) would allow researchers to manipulate ants and
isolate each of these behavioral and physiological changes, helping to paint a
more complete picture of the zombie fungus.

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